1. Field of the Invention
The present invention relates to a (brightening) chemical polishing solution for a hardened steel article, and a method of chemically polishing the hardened steel article by using the solution.
The present invention can be applied to hardened steel articles having a complicated shape, e.g., hardened gears used in a transmission gear, a differential gear and the like, to improve the properties of these articles, such as the surface roughness, fatigue strength, and wear resistance thereof.
2. Description of the Related Art
Steel articles requiring a high strength, e.g., transmission gears of automobiles, are subjected to a case-hardening heat-treatment, particularly, a carburizing and quench hardening treatment, and a carburized and quench hardened layer formed in the surface portion of the steel article (gear) has a high hardness and a residual compressive stress which improve the fatigue strength and wear resistance of the article. Recently, as the power output by automobile engines is increased, a greater fatigue strength is required of such articles.
A carburized and hardened steel article, however, has an abnormal layer, regarded as an oxidized and non-martensitic layer, having a depth of from 5 to 50 .mu.m from the surface thereof, and as such an abnormal layer has a hardness lower than that of the normal hardened layer existing thereunder, and thus lowers the residual compressive stress at the top surface, the abnormal layer is a factor in the lowering of the fatigue strength; a large surface roughness is another factor in the lowering of the fatigue strength, whether or not the abnormal layer exists.
To improve the fatigue strength of steel articles, shot-peening has been adopted as an additional process giving a relatively high compressive stress to a surface layer having a depth of from 200 to 400 .mu.m from the top surface thereof. The residual compressive stress caused by the shot-peening has a peak value at from 10 to 100 .mu.m from the top surface which is lower than the peak value thereof at a portion above the former-mentioned position. According to the shot-peening process, the steel articles are bombarded with hard particles at a high speed, and thus surface damage is liable to occur. Furthermore, the abnormal layer of the carburized and hardened steel article is hardly removed by the shot-peening, and thus a portion thereof remains. Such damage and the remaining abnormal layer portion are liable to become initiation points of fatigue crack, and hinder a stable and marked improvement of the fatigue strength.
A mechanical polishing process for removing this abnormal layer has been proposed in, e.g., "A Process for Producing a High Strength Gear" (Japanese Unexamined Patent Publication (Kokai) No. 01-264727, published on Oct. 23, 1989), in which a steel article (gear) is subjected to a carburizing and quench hardening treatment, and shot-peening, and is then ground with a grinding wheel of cubic boron nitride. The high hardness of the hardened article, however, lowers the grinding efficiency of the mechanical grinding. In particular, articles with a complicated shape, such as tooth-roots of a gear required a fatigue strength can not be precisely ground, with high efficiency. On the other hand, electrolytic polishing has been proposed in, e.g., Japanese Unexamined Patent publication (Kokai) Nos. 62-24000 (published on Jan. 31, 1987), 02-129421 (published on May 17, 1990), and 02-129422 (published on May 17, 1990). According to the above Publication No. 62-24000 (Electrolytic Polishing Process of Gears), electrodes are arranged near the tooth-bottom of a carburized and hardened gear, and an electrolytic polishing solution is sprayed toward the tooth-bottom, to thereby etch the tooth-bottom only. In this case, it is necessary to change the position of the electrodes, depending on the shape of the steel article, to ensure a dimensional accuracy, and thus this electrolytic polishing device has a complicated structure. Furthermore, according to the above Publication Nos. 02-129421 and 02-129422 (High Strength Coil Spring and Method of Producing the Same), a spring of chromium-vanadium steel is quench-hardened, tempered and shot-peened, and then subjected to an electrolytic polishing treatment. In this case, surface damages are removed to attain a surface roughness (R.sub.max) of 5 .mu.m or less, but the accuracy of the spring is not so severe. If the methods of these publications apply to articles (e.g., gears) required of a strict accuracy, the problem pointed out in the above Publication No. 62-24000 also occurs.
Taking the above-mentioned conventional processes and disadvantages into consideration, the present inventors though investigated the use of chemical polishing process for polishing a hardened steel article. A chemical polishing process for steel articles was proposed by, e.g., U.S. Pat. No. 3369914 (Method of Chemically Polishing Iron, Zinc and Alloys thereof). USP 3914 uses an aqueous solution of hydrogen fluoride and hydrogen peroxide, a molecular ratio of hydrogen peroxide to hydrogen fluoride being between about 3:1 and 7:1, and states that a metal component part is immersed in this solution bath for 1 minute to obtain a shining surface of the component part. It is possible to apply this polishing process to a pretreatment for plating, a treatment for improving a corrosion resistance, and a brightening treatment, without considering the polishing rate or polishing amount, but if this process is applied to a precision polishing of articles such as hardened gears, requiring a precise dimensional accuracy, since USP 3914 does not disclose suitable conditions for such a precision polishing treatment, a person skilled in the art cannot apply this process to a final polishing of parts. Furthermore, since the molar ratio of hydrogen peroxide to hydrogen fluoride is large (3 to 7), the hydrogen peroxide in the solution is liable to decompose during its solution is not used with the result that expensive hydrogen peroxide is wasted and the polishing solution is not suitable for an industrial polishing treatment, from the viewpoint of solution stability.
Furthermore, regarding the shot-peening, a "Method of Treating a Surface of a Carburized and Hardened Layer" (Japanese Unexamined Patent Publication (Kokai) No. 62-203766 (published on Sep. 8, 1987) was proposed, in which a steel article (e.g., a gear) is carburized and hardened, an abnormal layer is removed by a chemical dissolving (etching treatment, and the article surface is then shot-peened. In this case, the chemical dissolution (etching) produces a surface roughness (R.sub.max) of several tens of micrometers, and the shot-peening reduces this roughness. Nevertheless, although the abnormal layer is removed, the shot-peening damages the article surface, and thus no remarkable improvement of the fatigue strength is obtained.